The present invention relates generally to a brake control system, and more particularly, to a system for estimating the brake-fluid pressure.
In recent years, vehicle-mounted braking devices have various functions added. Such braking devices are known, for example, in the form of an antiskid braking system for achieving shortening of the braking distance while maintaining the vehicle behavior stable by preventing wheel lock, a traction control system for preventing spin of driving wheels during vehicle acceleration and the like, a brake assist device constructed so that when the brake-fluid pressure operated by a driver is insufficient, this insufficiency of brake-fluid pressure is supplied to a wheel cylinder, and a vehicle-stability control system for securing the running stability of the vehicle by providing a braking force to the wheels regardless of presence of brake operation by a driver so as to remove oversteer or understeer of the vehicle.
With the above braking devices, accurate grasp of the fluid pressure within the wheel cylinder and that produced by operation of a brake pedal will allow more precise control of a driving time or the like of a control valve for controlling the fluid pressure, leading to further enhanced control accuracy of the braking devices. One approach is to provide a fluid-pressure sensor to a master cylinder or a wheel cylinder so as to measure the fluid pressure within the master cylinder or the wheel cylinder. However, this approach involves an increase in manufacturing cost of the device.
Due to such problem of manufacturing cost, another approach is proposed wherein only the fluid pressure within the master cylinder is measured by the fluid-pressure sensor, and calculation is made as to how this pressure varies in accordance with a fluid-pressure model of the braking device to estimate the fluid pressure within the wheel cylinder. Specifically, a variation in the fluid pressure calculated in accordance with a drive signal for the braking device is added to an estimated fluid pressure (previous value) within the wheel cylinder obtained using the fluid pressure within the master cylinder, thus estimating a current value of the fluid pressure within the wheel cylinder.
However, as will be described later, the latter approach raises a problem that an estimated fluid pressure within the wheel cylinder exceeds an actual fluid pressure.